Need a more detailed timeline. How long have you had them? How your post reads is you got them in, have had them two days, did a water change the day after you got them and the next day two died. If that's how it happened not only were they stress from shipping (whether by mail or from the store to your home), but they were introduced to new parameters then you did a water change and possibly altered the parameters again. If that's the case, stress would be the one thing I'd look at that would cause their death. Or premature molting due to change in parameters. And PFRs aren't CRS so even if PFRs are doing great, that really doesn't matter in the grand scheme of things when you are dealing with a different shrimp entirely. I've had OEBTs drop like flies while Supremes were berried and flourishing...doesn't matter.
So yeah, how long have you had them, how long did you acclimate them, how old were they when you got them, etc. We just need more info. And at the end of the day, sometimes shrimp just die...it happens